Perspective pubs.acs.org/acscatalysis
Metal−Metal Bonds in Catalysis Ian G. Powers and Christopher Uyeda* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States ABSTRACT: Transition metals can assemble to form multinuclear complexes by engaging in direct metal-to-metal interactions. Metal−metal covalent bonds provide a large perturbation in electronic structure, relative to mononuclear metal ions, and the unique properties of these dinuclear fragments can be harnessed in a broad range of applications for example, as chromophores in photochemical processes, redox centers in molecular electronics, or structural elements in metal−organic materials. There is a growing body of evidence that metal−metal bonds may also be formed under conditions relevant to catalysis and play a key role in transformations that were previously assumed to only involve mononuclear species. These findings have stimulated interest in characterizing multinuclear reaction pathways and developing well-defined multinuclear platforms as catalytic active sites. In this Perspective, we present case studies in this emerging area of catalysis research, emphasizing the impact of metal−metal bonding in either enhancing or depressing the rate and/or selectivity of a catalytic organic transformation. KEYWORDS: catalysis, metal−metal bonds, cooperativity effects, multinuclear complexes, organometallic chemistry
1. INTRODUCTION Transition-metal catalysis has reshaped the landscape of organic synthesis, providing access to powerful bond constructions that facilitate the preparation of valuable compounds.1 It is now common to find transition-metal-catalyzed reactions designed into synthetic routes targeting the most complex secondary metabolites produced in nature.2 In addition, robust catalytic coupling reactions are routinely being applied in the pharmaceutical industry to generate libraries of lead compounds for drug discovery.3 These applications, and many others, capitalize on ongoing efforts by organometallic chemists to advance new catalytic platforms that offer significant improvements to established reactions or exhibit unprecedented reactivity. Many of the earliest homogeneous transition-metal catalysts were relatively simple coordination complexes, often comprising a single metal center bound only to the organic components being incorporated into the reaction product. For example, Co(CO)3H was introduced by Roelen in 1938 as the first catalyst for alkene hydroformylation.4 In addition, homoleptic Ni(C2H2)n species are the proposed catalytic intermediates in Reppe’s seminal acetylene cyclotetramerization system.5 Supporting ligands have since been established as a defining feature of transition-metal catalysts, providing access to additional parameters that can be used to optimize the rate and selectivity of a reaction.6 Over a significantly longer time scale, catalysts in nature have also evolved to use transition metals. Despite the analogy between transformations carried out by synthetic and biological catalysts, the multifunctional architectures of metalloenzyme active sites present a striking contrast to the comparatively simple structures favored by synthetic chemists. Inorganic © 2016 American Chemical Society
cofactors commonly incorporate multiple metals that function in concert to bind substrates and manage electron equivalents.7 Biological ligands can be redox-active and have a tendency to enforce weak-field environments, allowing catalysts to adopt high spin states or even simultaneously populate multiple spin states.8 Secondary coordination sphere interactions, such as hydrogen bonding, play a key role in stabilizing substrate− catalyst complexes.9 Insights into these bioinorganic systems have motivated a corresponding effort among organometallic chemists to incorporate these multifunctional features into fully synthetic systems.10 In this Perspective, we discuss historical and recent cases where dinuclear complexes featuring direct metal−metal interactions are implicated in a catalytic process. Metal−metal bonds have been a subject of significant research interest over the past half century;11 however, reports describing their applications in catalysis are relatively sparse.12,13 Nevertheless, many of the metal−ligand combinations used in catalytic reactions can support the spontaneous formation of dimeric or higher-order complexes. Indeed, metal−metal bonded species have recently been isolated from catalytic reactions that were previously assumed to only involve mononuclear complexes. For example, mechanistic studies of Pd-catalyzed cross-coupling and C−H functionalization reactions have uncovered Pd(I)14 and Pd(III)15 dimers as viable catalytic intermediates. In tandem with these studies, new multinucleating ligands have enabled unparalleled synthetic access to a broad range of stable metal−metal bonds that can be crystallographically Received: September 20, 2016 Revised: December 2, 2016 Published: December 9, 2016 936
DOI: 10.1021/acscatal.6b02692 ACS Catal. 2017, 7, 936−958
Perspective
ACS Catalysis characterized and studied spectroscopically. The emergence of these platforms presents unique opportunities for rationally advancing new classes of catalysts that exhibit fundamentally different behavior from the mononuclear systems that are currently being utilized in reaction methodology (see Figure 1).
Figure 2. A qualitative orbital model for the quadruple bond in [Re2Cl8]2−.17
Figure 1. Additional parameter space in catalyst design introduced by metal−metal bonding.
In this Perspective, we highlight cases where metal−metal bonds are thought to be involved in a catalytic organic transformation, placing particular emphasis on systems with substantial mechanistic data that correlate the presence of the metal−metal bond with catalytic function.
2. COORDINATION COMPOUNDS CONTAINING METAL−METAL BONDS The privileged role of transition metals in catalysis can be attributed to their ability to use d-orbitals to form stable covalent bonds with reactive organic fragments. The same electronic properties that underlie metal−ligand bonding also enable the formation of direct metal-to-metal interactions.12d Indeed, while any of the metallic elements could, in principle, participate in metal−metal bonding, a significant majority of all known cases involve two transition metals. The synthesis and characterization of Re2Cl82− (15) represented a landmark achievement in the field of inorganic chemistry, stimulating extensive research aimed at understanding the nature of the metal−metal bond.11a,16 In many early case studies of metal−metal bonding, the presence of bridging ligands created considerable ambiguity, with regard to distinguishing between true metal−metal interactions and ligand-enforced proximity. The Re−Re bond in Re2Cl82− was particularly significant, because it was both unsupported and exceedingly short. In order to rationalize the Re−Re bond length and the eclipsing conformation of the chloride ligands, Cotton developed a bonding model that invoked a metal− metal quadruple bond (Figure 2). The involvement of δinteractions in metal−metal multiple bonds is a unique property of transition metals with no analogue in main group elements.17 An expansive literature surrounding the synthesis and characterization of different metal−metal bonding motifs has since been developed.12d There are now over 40 000 examples of multinuclear complexes featuring close metal−metal contacts compiled in the Cambridge Structural Database.18 Metal−metal bonds can span a broad spectrum of polarities and bond orders (Figure 3a). Symmetrical homodinuclear complexes necessarily contain nonpolar metal−metal bonds, whereas heterobimetallic complexes, pairing early metal ions with low-valent late metals, possess strongly polarized bonds.12a Metals can form short and strong multiple bonds.11b Synthetic access to quintuple bonds
Figure 3. (a) Complexes containing covalent metal−metal bonds organized according to polarity and bond order.19a,22 (b) Complexes containing weak metal−metal interactions (bond orders 99% after 24 h (see Figure 32). By comparison, the Ni/In catalyst 67-In is significantly less active, and complexes 66 and 67-Al, which do not bind H2, afford no hydrogenation reactivity. These results highlight the high degree of control that can be obtained in Ni hydrogenation catalysis by tuning the strength of an ancillary Ni−Lewis acid interaction. 7.7. A Zr/Co Catalyst for Hydrosilylation. Thomas demonstrated that the Zr/Co complex 1545 functions as an efficient catalyst for the hydrosilylation of ketones, providing silyl ether products (see Figure 33).101 The reaction is particularly noteworthy from a mechanistic perspective in that it appears to circumvent standard silane activation pathways such as Si−H oxidative addition, which is a key step in the Chalk−Harrod mechanism.102 When complex 15 is treated with PhSiH3 (1.0 equiv), no reaction is observed; however, 15 reacts cleanly with benzophenone to generate the dimeric complex 68. The structure of the dimer (68) is reminiscent of Gomberg’s dimer and presumably arises from a C−C coupling reaction of two Zr-bound ketyl radicals (69). Based on this reactivity, Thomas proposed a catalytic mechanism in which the initial coordination of the ketone to Zr is accompanied by electron transfer to generate a ketyl radical. Addition of the silane could then proceed via a concerted or stepwise process to generate the silyl ether product, which dissociates from the catalyst. A unique feature of this mechanism is that substrate binding occurs only at Zr; however, the redox chemistry is distributed between the two metals. A Zr(III)/Zr(IV) redox cycle in a mononuclear catalyst would be inaccessible under typical hydrosilylation conditions because of the driving force for Zr to maintain its highest possible oxidation state; however, the reducing Zr−Co bond can formally function as a one-electron
to donation from a filled Pd(dz2) orbital into an empty orbital on the d0 Ti center. Mayer atomic bond orbital analysis indicated a Ti−Pd bond order of 0.29. Michaelis and Ess reported a combined experimental and computational analysis of this system to evaluate the relative importance of a direct Ti−Pd covalent interaction versus an indirect electrostatic effect of Lewis acid coordination to the phosphinoamide (Figure 31).98 In the calculated transition structure for amine addition, the Pd−Ti distance undergoes a contraction relative to the ground state, suggesting a strengthening of the interaction. In addition, when Ti was geometrically constrained to be coplanar with the two P−N bonds (65), thus breaking the Pd−Ti interaction, the activation barrier for amine addition increased by 4.5 kcal/mol. Complex 64 was selected as a monopalladium comparison compound and found to exhibit an activation barrier that was even higher in energy than that for 65 (by 3.5 kcal/mol). Experimentally, the addition of Et2NH to methallyl chloride using catalyst 64 reaches a yield of only 51% after 3 h. Using the highly active Ti/Pd catalyst 62, Michaelis described a general synthetic method for allylic aminations that can accommodate hindered secondary amine nucleophiles, such as 2,2,6,6,-tetramethylpiperidine, which are unreactive using monopalladium catalysts.99 7.6. Metal−Lewis Acid Dative Interactions in NiCatalyzed Hydrogenation. Lu examined the effect of a metal−metal dative interaction in Ni-catalyzed hydrogenation reactions (Figure 32) within an isostructural series of 3-fold symmetric complexes featuring Ni−M bonds (M = Al, Ga, or In, 67).100 As the size of the metal atom was increased across the series (Al < Ga < In), a decrease was observed in the ratio of the Ni−M bond length to the sum of the covalent radii. This structural trend was attributed to better overlap of the larger group 13 cations with the soft Lewis basic Ni center, leading to a stronger dative interaction. This effect is also directly reflected in the one-electron oxidation potentials, which shift toward 951
DOI: 10.1021/acscatal.6b02692 ACS Catal. 2017, 7, 936−958
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ACS Catalysis
reductant, generating the ketyl radical character that is necessary for addition of the silane. 7.8. Alkyne Cyclotrimerizations Catalyzed by Co2(CO)8. The Co2(CO)8-mediated cyclotrimerization of alkynes, first reported in the early 1960s by Hubel, is a seminal example of a catalytic process thought to occur at a dinuclear active site (Figure 34).103 Diphenylacetylene reacts at 100 °C in the presence of 3 mol % Co2(CO)6(PhCCPh) (70), providing hexaphenylbenzene in up to 95% yield. Notably, the catalytic process was observed to occur at significantly lower temperatures and in higher yields than with other homoleptic carbonyl complexes of Fe, Ni, Mn, Mo, or W. Although the mechanism of cyclotrimerization is not fully understood, it necessarily involves the sequential coupling of three equivalents of alkyne at the catalyst (Figure 34). With regard to the initial alkyne adduct, myriad examples of Co2(μalkyne) complexes have been isolated, and they uniformly exhibit a perpendicular coordination of the alkyne across the Co−Co bond. The addition of the second alkyne equivalent could potentially yield either an η4-cyclobutadiene (73) or a cobaltacyclopentadiene (71) complex. In 1969, Ehmann excluded the formation of cyclobutadienes using isotopic labeling experiments and observed product isotopomers that were consistent with metallocyclic intermediates.104 It was not until 1999, however, that Spicer presented a general synthesis of isolable, structurally characterized, and catalytically relevant dicobaltacyclopentadiene complexes (71).105 In all isolated structures, the two Co centers occupy unique environments, with one Co atom incorporated into a metallocyclopentadiene ring and the other engaged in a secondary η4-interaction with the dienyl π-system. A somewhat longer dispute persisted over the identity of the intermediate resulting from the addition of the third alkyne equivalent (Figure 34). In several stoichiometric and catalytic processes, the formation of “flyover” complexes (74) was
Figure 33. A Zr/Co catalyst for ketone hydrosilylation proceeding through a Zr-bound ketyl radical intermediate.101
Figure 34. Proposed mechanism for the cyclotrimerization of alkynes by Co2(CO)8 involving dinuclear catalytic intermediates. 952
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Figure 35. Comparison of the Ni2 catalyst 18 and mononickel catalysts (75−78) in the cyclotrimerization of methyl propargyl ether.108
7.9. Evaluating Nuclearity Effects in Alkyne Cyclotrimerizations. In order to probe the effect of nuclearity on catalytic efficiency in alkyne cyclooligomerizations, Uyeda utilized the [i‑PrNDI]Ni2(C6H6) complex 18, which functions as a well-defined, low-valent platform for studying reactions at a metal−metal bond (see Figure 35).108 The Ni2 complex 18 effects the rapid and selective cyclotrimerization of terminal alkynes, forming 1,2,4-substituted arene products. By comparison, a variety of mononickel catalysts bearing analogous Ndonor chelating ligands (75−78) were found to exhibit significantly lower activity and generally afforded mixtures of products, including the 1,3,5-regioisomer, cyclooctatetraenes, and linear oligomers. The most striking effect was observed in the cyclotrimerization of methyl propargyl ether: the Ni2 catalyst 18 (1 mol % loading) provides exclusively the 1,2,4substituted arene 79 (98% yield) within minutes at room temperature. Under the same set of conditions, mononickel catalysts uniformly afforded low conversions and yielded
